1. Field of the Invention
This invention relates to detection systems, and more specifically to an inexpensive short-range detection system for locating reinforcing steel rods, pipes, and other nearby objects positioned behind or within a volume of, e.g., concrete, soil, or wood.
2. Description of Related Art
A requirement exists for a cost-effective system capable of locating reinforcing steel rods (rebar), pipes, bubbles, and other objects buried in concrete or soil, or hidden behind walls or other structures. Various devices and systems currently exist to locate these various objects, but all are either limited in capability or very costly.
To overcome disadvantages of other available systems, Zircon Corporation of Campbell, Calif., developed an improved radar system for locating objects behind or within a volume of material. That radar system, described in the above-incorporated U.S. Pat. No. 5,543,799 entitled "Swept Range Gate Radar System for Detection of Nearby Objects," relates to the ultra-wide band (UWB) radar technology developed by the Lawrence Livermore National Laboratory (LLNL). (For more information on the UWB radar technology developed by the LLNL, see U.S. Pat. No. 5,457,394 issued Oct. 10, 1995 entitled Impulse Radar Studfinder, U.S. Pat. No. 5,361,070 issued Nov. 1, 1994 entitled Ultra-Wideband Radar Motion Sensor, and U.S. Pat. No. 5,345,471 issued Sep. 6, 1994 entitled Ultra-Wideband Receiver, all issued to McEwan and are incorporated by reference.
The radar system described in the above-referenced Zircon patent application transmits a pulse and senses a return echo. The radar system then provides indications of (1) the strength of the return echo, and (2) the time lapse between transmitting the pulse and receiving the return echo. The strength of the return echo provides an indication of the size and material of the object reflecting the signal, while the time lapse provides an indication of the distance (i.e., range) between the radar system and the object.
The aforementioned radar system is contained in a radar unit that may be moved over a surface of a volume of material to determine the presence and location of objects within or behind the volume (e.g., reinforcing steel, such as "rebar," may be located within a concrete wall). All physical movement of the radar unit over the surface being scanned is performed by a human operator, with any given display presentation being uniquely associated with a given position of the unit on the surface. Thus, the operator has physical control of the X and Y coordinates (i.e. the surface of the volume being scanned), while the radar unit scans into the volume along a Z axis normal to the surface. Any change in amplitude on the display can now be associated with a particular point within the volume of the object, the point being uniquely specified by X, Y, and Z coordinates. In addition, the amplitude of the echo gives an indication of the size and material of the object from which the pulse is reflected.
FIGS. 1A, 1B, 1C, and 1D show a planar graphics display 14 with several exemplary display examples. FIG. 1A depicts a planar graphics display that has two axes of information: range (Min to Max) and amplitude A (zero to Max on either side of the centerline). The display 14, as physically attached to the unit, is typically parallel to the surface being scanned when the unit 15 is in use. The display 14 thus provides a representation of the cross-section of the volume being scanned at that physical location on the surface. The display of amplitude information is in one embodiment "mirror imaged" about the center line shown to eliminate any X-Y bias.
In FIG. 1A, the volume being scanned contains only homogeneous material within the scan range of the unit. Thus, display 14 provides no indication of a subsurface object.
FIG. 1B shows the detection of metal rebar at depth D1 into the volume and depicted on the display at position D1' indicating the depth. FIG. 1C is similar to FIG. 1B, but with the rebar at a greater depth D2, shown on display 14 at position D2'.
FIG. 1D illustrates a possible source of error in the radar system of unit 15. In FIGS. 1B and 1C, the unit 15 was centered below the rebar, thus giving an accurate measure of the respective depths D1 and D2. In contrast, FIG. 1D shows a case where the unit 15 is not centered above the rebar, and therefore gives an erroneous indication of the depth D2.
The depth D2 is the same in FIGS. 1C and 1D. However, because the offset of FIG. 1D increases the distance traveled by the pulse, the measured depth D2" of the unit 15 in FIG. 1D is greater than the true measured depth D2' of FIG. 1C. To achieve an accurate measure of depth and to properly center the unit 15, the operator must move the unit 15 across the surface until the depth reading is minimized.
Unfortunately, it can be difficult to precisely determine the point at which the distance measurement is minimized, and therefore to determine when the unit 15 is centered on the object being located (e.g., the rebar). Moreover, the process of centering becomes more difficult as the depth of the object increases. Therefore, a radar unit that is more easily centered over subsurface objects, thereby allowing a user to locate such objects with greater precision, would be advantageous.